Technique for adjusting temperature coefficient of resistance of tantalum aluminum alloy films

ABSTRACT

FFIG-01 A TECHNIQUE FOR ADJUSTTING THE TEMPERATURE COEFFICIENT OF RESISTANCE OF SPUTTED TANTALUM ALUMINUM ALLOY FILMS TO A VALUE WITHIN THE RANGE OF -100 TO 300 P.P.N./*C. INVOLVES HEAT TREATING THE SPUTTERED FILM AT TEMPERATURES RANGING FROM 650-950*C. IN AN AMBIENT IN WHICH THE MAXIMUM PARTIAL PRESSURE OF OXYGEN IS 1C10-**5 TORR. THE DESCRIBED TECHNIQUE PERMIS THE REALIZATION OF TEMPERATURE COEFFICIENTS OF RESISTANCE OF APPROMIMATELY ZERO, THEREBY MAXIMIZING THE DEVICE APPLICATIONS OF SUCH FILMS.

June 1973 J. M. CHILTON ETAL 3,738,9 9

TECHNIQUE FOR ADJUSTING TEMYEHA'I'UHIFI UOK'J'P'P'ICU'J'NT ()l" REISTANCE OF TANTALUM ALUMINUM ALLOY FILMS Filed April 13, 1972 TCR- PPM/"C TEMPERATURE C United States Patent TECHNIQUE FOR ADJUSTING TEMPERATURE COEFFICIENT 0F RESISTANCE 0F TANTALUM ALUMINUM ALLOY FILMS John Michael Chilton, Coopersburg, and Donald Jaife, Emmaus, Pa., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Continuation-impart of abandoned application Ser. No. 214,205, Dec. 30, 1971. This application Apr. 13, 1972, Ser. No. 243,630

Int. Cl. C23c 15/00 US. Cl. 204-37 R 5 Claims ABSTRACT OF THE DISCLOSURE A technique for adjusting the temperature coefficient of resistance of sputtered tantalum aluminum alloy films to a value within the range of l00 to 300 p.p.m./ C, involves heat treating the sputtered film at temperatures ranging from 650-950" 'C. in an ambient in which the maximum partial pressure of oxygen is 1 10- torr. The described technique permits the realization of temperature coefiicients of resistance of approximately zero, thereby maximizing the device applications of such films.

This invention is a continuation-in-part of copending application Ser. No. 214,205, filed Dec. 30, 1971, now abandoned.

This invention relates to a technique for adjusting the temperature coefiicient of resistance of tantalum aluminum alloys to a value within the range of -100 to +300 p.p.m./ C. More particularly, the present invention relates to a technique for adjusting the temperature coefficient of resistance of tantalum aluminum alloys containing from 25-60 atom percent aluminum by heating techniques.

DESCRIPTION OF THE PRIOR ART Miniaturization of components and circuitry, coupled with the increasing complexity of modern electronic systems, have created an unprecedented demand for reliability in thin film components. For many years, the requirements of stability, precision and miniaturization have been fulfilled simultaneously by the use of tantalum components wherein elemental tantalum or a compound thereof has been utilized in the form of a thin film. Recently, materials capable of competing with tantalum or its compounds were described in the literature. These materials comprise tantalum aluminum alloys and are typically obtained by cosputtering a thin film of the alloy containing from 25-60 atom percent aluminum upon a substrate member, so resulting in the formation of a highly stable thin film resistor manifesting a sheet resistance within the range of 25-1000 ohms/ square. The resistors so obtained can be readily and precisely adjusted to value by anodization and therefore their utility is suggested for many circuit and RC network applications. However, a potential limitation of the noted materials resides in the fact that the temperature coefficient of resistance of tantalum aluminum resistors typically ranges from 1l0 to 135 p.p.m./ C., thereby excluding their use from device applications requiring materials manifesting a temperature coefiicient of resistance near zero. Accordingly, Workers in the art have recently focused their interest upon this specific limitation with a view toward maximizing the potential of tantalum aluminum alloys. Although similar efforts were directed toward solving this problem in tantalum nitride resistors, no successful efforts have been reported heretofore with respect to tantalum aluminum alloys.

SUMMARY OF THE INVENTION In accordance with the present invention, the prior art limitation alluded to above has been successfully obviated. Briefly, the inventive procedure involves adjusting the temperature coefiicient of resistance of the sputtered tantalum aluminum by heating the as-sputtered film to a temperature ranging from 650-900 C. in an ambient in which the maximum partial pressure of oxygen is 1X10" torr. The described technique permits the realization of temperature coefiicient of resistance values in tantalum aluminum alloys of approximately zero and also over the range of l00 to +300 p.p.m./ C., so maximizing the availability of these materials for device applications.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fully understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

The figure is a graphical representation on coordinates of temperature coefficient of resistance in parts per million per degree centigrade against temperature in degrees centigrade showing the variations in temperature coeflicient of resistance of various tantalum aluminum alloys after heating in vacuum for one hour.

DETAILED DESCRIPTION OF THE INVENTION The inventive process contemplates the use of a substrate upon which the resistor of interest is to be produced. Suitable substrate materials are those which conform to the requirements imposed by the various process stages. It is preferred that the substrate be possessed of a smooth surface which is completely free from sharp changes in contour and should be a material which is able to withstand temperatures as high as 900 C. All types of refractory materials such as quartz, ceramics and high melting materials meet these requirements.

The substrate so selected is initially cleansed by means of any conventional cleaning techniques, the choice of a particular cleansing agent being dependent upon the composition of the substrate itself. Thereafter, the substrate is placed in a sputtering apparatus suitable for the deposition of tantalum aluminum alloy films having a desired aluminum content in the range of from 25-60 atom percent.

The conditions used in cathodic sputtering as employed herein are known. By employing a proper voltage, pressure and spacing of the various elements within the vacuum chamber, a layer of a tantalum aluminum alloy is deposited upon the substrate in the desired configuration and thickness.

Following the sputtering step, the as-sputtered tantalum aluminum alloy film is heated at temperatures ranging from 650-900 C. for a time period ranging from 1 minute to 300 minutes in an ambient in which the maximum partial pressure of oxygen is l 10- torr for the purpose of adjusting the temperature coeflicient of resistance to the desired value. The noted temperatures and time periods are not considered absolute but are dictated by practical considerations. Accordingly, the use of temperatures for time periods less than the noted minima will not yield any appreciable change in temperature coefficient of resistance whereas exceeding the maximum results in material and apparatus degradation.

As indicated, the heating step may be conducted in any ambient (non-reactive) having a maximum partial pressure of oxygen of 1 10- torr. At pressures greater than 1 1O torr, considerable oxidation of the film occurs during the heat treatment, resulting in high contact resistance and high noise levels in resistors generated therefrom. The desired ambient may conveniently be an inert gas, such as nitrogen, argon, neon and the like, or a vacuum. The critical feature in each case is that the oxygen partial pressure not exceed 10* torr.

In the fabrication of resistors in accordance with the invention, the resultant structure subsequent to heating may be anodized until a desired value of resistance is attained, thermally preaged or treated in any manner consistent withits ultimate use.

An example of the present invention is described in detail below. The example is included merely to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

Example A 99.5 percent aluminum oxide substrate was selected for use herein. The substrate was initially fire-cleaned at 1200" C. using conventional procedures. Films were deposited by two different techniques during the course of the investigation. Conventional DC sputtering using a composite tantalum-aluminum cathode was employed to obtain compositions close to 50 atom percent aluminum. The cathode consisted of 250 aluminum discs inserted into a 35.6 cm. diameter tantalum plate such that the discs were coplanar with the tantalum plate at the sputtering surface. Sputtering was performed at 5 kv. with a current density of 0.25 ma./crn. using an anode to cathode distance of 9 cm. An argon pressure of 35 millitorr was employed. A presputtering period of about 45 minutes during which time the substrates were shielded by a mechanical shield preceded deposition of the alloy films.

The other sputtering technique used was an AC rod sputtering process. In this case the cathode and anode consisted of a planar array of hollow rods of tantalum and aluminum alternately positioned. Each set of rods was connected to an electrically isolated high voltage AC power supply, such that each set alternated as anode and cathode during an AC cycle. A third electrode, the field bias plate, was used to vary the electrical characteristics of the plasma. Compositional variation could be accomplished electrically by adjustment of a series DC bias applied to the tantalum rods, making this technique particularly attractive for depositing films of different aluminum content in the present work. Depositions were made with an AC RMS voltage of 5 kv. and an RMS current of 400 ma. The DC bias on the tantalum rods was varied from to 1400 volts, depending on the composition of the film required, with the field bias plate held at -200 volts throughout. Sputtering was carried out with an argon pressure of 40 microns and a pre-sputter period of about 30 minutes.

Subsequent to sputtering, the temperature coeflicient of resistance of the sputtered tantalum aluminum alloys was measured and thereafter the alloys were heated to a temperature ranging from 650-900 C. for one hour in a vacuum maintained at a pressure between 1X10 and 1X10 torr, the temperature coefficient of resistance being monitored subsequent to the heating process. The results of these measurements are reflected in the data plotted graphically in the figure, which is discussed below.

The results of the vacuum heating process are shown graphically in the figure, which shows variations in temperature coeflicient of resistance as a function of temperature during the heating of tantalum aluminum alloys hav ing aluminum contents ranging from 25-60 percent. It is noted by reference to the figure that the temperature coeflicient of resistance of the as-sputtered film is approximately p.p.m./ C. and in each situation as the heating continues over the temperature range of 650-900 C. the temperature coetficient of resistance becomes less negative and ultimately changes from negative to positive values, so indicating the availability of a zero temperature coefficient of resistance material. As heating increases at higher temperatures this parameter continues to increase positively to approximately 300 p.p.m./ C.

What is claimed is:

1. Technique for adjusting the temperature coefiicient of resistance of a thin film resistor consisting of a sputtered tantalum aluminum alloy film containing from 25- 60 atom percent aluminum upon an insulating substrate, which comprises heating the sputtered film to a temperature Within the range of 650-900 C. for a time period ranging from 1 to 300 minutes, the shorter time periods corresponding with the higher temperatures and the converse, in an ambient selected from the class consisting of inert gas and a vacuum, in which the maximum partial pressure of oxygen is 1X10 torr.

2. Technique in accordance with claim 1 wherein said alloy is heated at about 800 C. for three hours in a vacuum maintained at 10- torr.

3. Technique in accordance with claim 2 wherein said alloy film comprises 50 atom percent aluminum.

4. A stable metal film resistor including successively a non-conducting substrate and a thin film consisting essentially of a tantalum-aluminum alloy consisting of from 2560 atom percent aluminum and manifesting a temperature coefficient of resistance of Zero, said metal film resistor having been heat treated within the range of '650-900" C. for a time period ranging from 1 to 300 minutes, the shorter time periods corresponding with the higher temperatures and the converse, in an ambient selected from the class consisting of inert gas and a vacuum in which the maximum partial pressure of oxygen is 1x 10 torr.

5. A stable metal film resistor in accordance with claim 4 wherein aluminum is present in said alloy in an amount of 50 atom percent.

References Cited UNITED STATES PATENTS 3,159,556 1'2/1964 McLean et al 204-37 R 3,320,500 5/1967 Axelrod et a1. 317-258 3,627,577 12/1971 Steidel 204-192 FOREIGN PATENTS 1,067,831 5/ 1967 Great Britain 204-192 1,132,903 11/1968 Great Britain 204-192 OTHER REFERENCES Proceedings 1970 Electronic Components Conference, 1970, p. 58.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

